The school district released a statement saying 25-year-old Brian Smart of Hatboro suffered a heart attack during the race and died.
More information will be released as the district receives it.
Smart's father asked Upper Darby athletic director Frank Nunan to pass along a statement: "It is obvious that Brian was beloved by the students that he worked with, and we've been receiving messages all morning about his passing," read a statement posted by the district.
"Please join the entire Upper Darby School District community in extending our thoughts and prayers to Brian Smart’s family during this time."
A post shared by Upper Darby School District (@udsdofficial) on May 6, 2019 at 10:23am PDT
"He was a good runner and in great shape," Nunan said. "His brother-in-law told me that they did work on him — the Philadelphia paramedics — and they took him to the hospital and couldn't revive him."
The school district added that Smart, an alumnus of Temple University, was loved by the students he worked with, and the district has been receiving messages all day about his death.
Nunan added that students have created a memorial outside of Smart's office with flowers, notes and drawings.
"The students have mentioned in all their letters and posts out here just how personable he was to them," he said. "He didn't just care about their injury — he cared about them. He was motivating them for the next contest, and he reached them."
Smart — who had just moved into a Horsham apartment in March with his fiancee — is the third and youngest person to die in the 10-mile race since it began in 1980.
Funeral arrangements have not yet been finalized.